1. (Field of Industrial Applications)
The present invention relates to a self-threading screw, such as a self-drilling screw or a tapping screw, having a head of synthetic resin.
2. (Prior Art)
There has been provided a "self-drilling screw" which self-threads as self-drilling a workpiece having no pilot holes with its drilling portion formed in the forward end. Also, a "tapping screw" is well known which self-threads for tapping the holes preliminarily formed in workpieces. These screws are called as "self-threading screws".
Particularly in the field of construction industry, such a self-threading screw is widely used for fastening an attachment such as a panel to an objective member since no particular preparation for fastening is required on any of the attachment and objective member. More specifically, the former "self-drilling screw" requires no pilot hole in objective members while the latter "tapping screw" requires pilot holes in objective members but no female threading in the holes. These arrangements facilitate field screwing jobs for various industrial applications.
A major disadvantage of the self-threading screw of this type is however that as the screw is made of metal and rigidly heat treated to ensure its functions described above, its head can rust after attachment and thus, its appearance will be spoied. Also, when used for fastening such a colorful attachment, e.g. a color panel, to an objective member, the screw may have visual defect with the metallic-colored head, which is substantially exposed on the surface of the attachment, failing to match the colors on the panel.
To overcome such problems, there has recently been proposed a modified metal self-threading screw having a head formed of synthetic resin material.
Such a prior art self-threading screw having a synthetic resin head is manufactured by known technics of inserting a small tubular head portion of the metallic screw body into a set of molds and injecting synthetic resin material so that the small tubular head portion can be covered with the material to form a synthetic resin head. The small head portion is arranged having projecting ribs so as to retain the synthetic resin head securely. The synthetic resin head has a socket wrench engaging side formed on the circumferential periphery thereof and arranged into a polygonal shape, e.g. a hexagon.
When this prior art self-threading screw is screwed into a workpiece, its synthetic resin head, although made of particular rigid material having an advanced physical property, such as nylon, may be injured in the polygonal engaging side upon failing to stand against the rotational torque given by a socket wrench and thus, slip off an engaging part of the socket wrench which then turns ineffectively. This results from the fact that since a great rotational torque is needed to force a screw into a workpiece, the socket wrench is commonly turned by a power tool such as an electric tool or a pneumatic tool, so as to provide a considerably great load on the engaging side of the synthetic resin head.
This ineffective turning may occur not only when the self-threading screw has been screwed up in a workpiece but also during the screwing of the self-threading screw into a workpiece. If the socket wrench starts turning ineffectively during the screwing, it will also be impossible to turn the screw in the reverse direction for removal.